The present invention relates to an electronic flash device capable of automatically controlling the quantity of a flash of light.
The conventional electronic flash devices capable of controlling the quantity of a flash of light may be generally divided into two types. In one type, an electronic flash device is organically combined with a camera so that the quantity of a flash of light can be controlled in response to electrical signals representative of exposure factors and in the other type, the quantity of a flash of light is controlled in response to signals which are derived from an arithmetic circuit incorporated in the flash device and which represent exposure factors.
In general, the electrical signal derived from the camera or the arithmetic circuit represents an f number or aperture selected. Therefore, the quantity of a flash of light is automatically controlled in response to a selected f number.
That is, with the conventional electronic flash devices capable of controlling the quantity of a flash of light, a signal from a camera or an arithmetic circuit is converted into a voltage signal which is applied as a reference voltage to one of the two input terminals of a comparator. Another voltage obtained by the integration of the photocurrent flowing through a photosensor is applied to the other input terminal. The two voltage signals are compared and in response to the difference between them, the quantity of a flash of light is controlled. This is the most widely used system for controlling the quantity of a flash of light.
Such control system as described above, however, has the problem that the quantity of a flash of light cannot be controlled in a plurality of steps because of a limited range of variation in reference voltage applied to the comparator. In other words, the conventional electronic flash devices cannot control the quantity of a flash of light depending upon a plurality of f numbers.